Fix formatting in example Makefile.

Make expects tabs, `:`s, or `=`s as separators. Make 3.81 (at least) fails
while executing the example Makefile as previously provided (with 4-space
indentation not tabs).
Add note to Readme.md re: Makefile indentation.

Even though commit 6b229252d3d fixes the indentation of the Makefile example
code snippet, GitHub converts the tab into 4-space indentation when it renders
the README. This commit adds a note explaining that you may hit an error if you
copy and paste the snippet, and if you do, you should replace the 4-space
indentation with tabs.
Merge pull request #842 from hangtwenty/master

Simple README change - Make example Makefile code snippet use tabs not spaces